This invention relates to an RFID tag that comprises an antenna and tag LSI, and more particularly to an RFID tag having a plurality of patch antennas of different sizes as tag antennas so that the RFID tag can be utilized in a plurality of areas using different transmission frequencies.
Conventionally, in the distribution industry, transportation industry and the like, one method that is widely used as a method of controlling and managing various kinds of product information, is a method in which barcodes are printed onto or stuck on a product itself or to the product box, and this barcode is read by a barcode reader. However, in the method of processing barcodes, the barcode reader must come in contact with the barcode, so the work of reading the barcodes is somewhat troublesome. Also, in a conventional barcode processing method there is a problem in that it is not possible to add or update new information to the barcode itself. Therefore, in recent years, instead of barcodes, a method of attaching RFID (Radio Frequency Identification) tags to products and the like, and reading the product information without contact by radio communication (electromagnetic coupling) is being demanded and also being put into practice. In an RFID tag, a radio communication function for transmitting information has been added to the function of an IC card, and comprises a nonvolatile memory that is capable of storing information, and does not have a battery (power supply section). Therefore, when a tag reading device reads information from the memory of the RFID tag without contact, it supplies power to the RFID tag by electromagnetic waves, and reads the information from the memory. By using an RFID tag, workability can be greatly improved, and by using technology such as a verification function and cipher function, it is possible to maintain excellent security.
FIG. 21 is a drawing explaining an RFID tag, where a reading device 1 sends a radio signal (electromagnetic wave) that is modulated with transmission data to the RFID tag 3 from an antenna 2. The antenna 3a of the RFID tag 3 inputs the received signal to a rectifying circuit 3b and modulation/demodulation circuit 3c. The rectifying circuit 3b converts the radio signal to a DC voltage and supplies the DC voltage to the modulation/demodulation circuit 3c and a logic circuit 32d, which acts as a power supply. The modulation/demodulation section 3c demodulates the control data that was sent from the reading device 1 and inputs the result to the logic circuit 3d. The logic circuit 3d performs logical processing according to the control data (commands); for example, it reads the information stored in the internal memory and inputs it to the demodulation/modulation circuit 3c. The modulation/demodulation circuit 3c uses the information that is input from the logic circuit 3d to modulate a carrier wave, and transmits that carrier wave to the reading device 1 from the antenna 3a. 
Various types of RFID tags have been proposed. As one of these, the inventors of this invention have proposed a UHF band RFID tag that comprises: a power-supply pattern section on which a tag LSI is mounted, a patch antenna that functions as a tag antenna, and a high-frequency coupling section that performs high-frequency coupling between the power-supply pattern section and patch antenna (refer to Japanese patent application 2006-216727). With this UHF RFID tag, the communication distance does not become poor even when the tag is stuck on a metal or liquid object, and it is not necessary to open holes up in the patch antenna for supplying power or to use an impedance conversion circuit, so this tag has the advantage that it can be made small and thin, however, there is a problem in that the frequency band is narrow, and the gain quickly becomes poor when the frequency shifts from the resonant frequency of the patch antenna. By reason of the narrow frequency band and poor gain characteristic, it is impossible to use the proposed UHF band RFID tag in a case where a wide frequency band is demanded. In other words, the transmission frequency differs according to country, for example, in Japan the transmission frequency used is 952 to 954 MHz, in the USA the transmission frequency used is 902 to 928 MHz, and in Europe the transmission frequency used is 869.4 to 869.65 MHz, so a RFID tag having a narrow frequency as described above cannot be applied to products or parts that are to be used in a plurality of countries to manage product information.
As prior art, there is a multi-band RFID tag that can be used in a plurality of frequencies (refer to Japanese patent application JP10-70411A). As shown in (A) of FIG. 22, this RFID tag comprises construction in which one H-type patch antenna 5 is replaced by a plurality of line elements 6 as shown in (B) of FIG. 22, where by changing the length of the line elements 6a, 6b, the tag is capable of corresponding to a plurality of frequencies (7.8 GHz, 10.8 GHz) as shown in (C) of FIG. 22. However, with the RFID tag of this prior art, it is necessary that the frequencies be sufficiently separated from each other, and there is a problem in that the tag cannot be used when the frequencies are relatively close. In other words, in the case of the frequencies used in Japan, the USA and Europe, where the frequencies are relatively close, the tag cannot be used.